In modern efforts to prefabricate structural elements, it has become common to use as inner or outer structural walls of buildings of all types, multilayer panels which can be provided as structural (load-bearing) or facing elements and which can comprise inner and outer reinforced-concrete slabs joined by at least one connector or anchor traversing the space between the slabs, this space being filled with an insulation or being reserved as an insulating air space or being a combination insulation and air space.
Such slabs are prefabricated and transported to the construction site where they are erected and anchored in place in the structure, serving as facade or structural elements.
Attention is directed to U.S. Pat. Nos. 3,757,482 and 3,996,713 and the publications mentioned in the files of these patents for an overview of sandwich slab construction techniques, anchor arrangements and the like.
The composite panels referred to above generally include flat arrays of reinforcing bars to which the connecting anchor can be affixed, and various anchor arrangements in the form of plates or membrane sheets. A tubular anchor can be provided at the centroid of at least a portion of the panel while the membrane or sheet anchors can be spaced about the centroid.
The high-stability composite panels can be load-carrying members as well as facing members and thus can be used for floors, roofs or walkway structures as well as for walls.
The thermal barrier or insulation between the two reinforced-concrete slabs may be of the single layer of multilayer type, with or without intervening trapped-air spaces, although the gap between the two plates may also be empty so that the insulation consists exclusively of a trapped-air space.
Connecting or main anchors, as this term is used for the purposes of the instant disclosure, are anchors which maintain a static connection between the reinforced-concrete inner plate and the reinforced-concrete outer plate and thus are correspondingly dimensioned.
Several such anchors can be provided for each panel, i.e. to connect each pair of coextensive slabs, although it is also possible to use a single connecting anchor which, as described, is advantageously located at the centroid of the panel.
These connecting anchors or compound anchors, because of the nature of their function, may be combined with auxiliary anchors such as hairpin anchors and the like which do not materially contribute to static support of one slab on the other. They serve primarily to discourage separation of the panels while torque and like stresses are absorbed mainly at the compound anchor. When reference is made herein to a compound or main anchor, therefore, it will be understood that this term can refer to the single anchor located at the centroid, or a plurality of anchors providing static support whether located at the centroid or not as long as they contribute to a significant degree to the static stability of the panel. A reference to an anchor in the singular will thus also be understood to be a reference to each anchor of a given panel assembly when more than one anchor is provided.
The compound anchor or anchors of the conventional structure are secured directly to the reinforcement members of the respective concrete plates by being tied, welded or linked thereto.
In the fabrication of the panels, the reinforcement of a first slab is assembled in a generally horizontal form and the anchor affixed thereto before pouring of the concrete of the first slab. The concrete is compacted by vibration and, after setting, means is applied to the slab to form the insulating gap. When the gap is filled with an insulation layer, this layer is applied so that the anchor projects therefrom. When the insulating space is empty, i.e. is to form a trapped-air space, a layer of sand can be applied to the first slab so that the anchor projects from this layer. The sand ultimately may be poured from the gap between the two slabs.
A second reinforcement structure is then assembled on this layer and connected to the anchor, whereupon the concrete of the second slab is poured and compacted by vibration.
This method of making facade panels has been found to give rise to several disadvantages.
When a planar concrete slab is fabricated in a horizontal form or bed, especially when vibration is used to compact the slab, there is a certain degree of separation of light components from heavier components. For example, the heavier components of the aggregate tend to sink to the bottom while the lighter components tend to be segrated at the upper surface of the slab. The upper surface therefore is found to have a higher concentration of water-rich cement while the lower layer has a greater concentration of heavier components of the aggregate and less water-rich cement. The slab, upon setting, has a higher density layer along the surface forming the underside which also may be of greater strength while the opposite face of the slab may have a lower density and less strength. The slab is not uniform throughout its cross section and deterioration can result.
Upon hardening and drying of the slab, moreover, the shrinkage effects on the two sides may differ with the upper layer shrinking more rapidly than the lower layer. When this occurs, the slab tends to bow and become downwardly convex. The convex surface, in panels of the type described, usually is the outer surface of the panel.
When the panel is formed from two slabs fabricated in this manner, both slabs have this tendency to bow. This bowing is highly undesirable and creates significant problems when the panels are used as facade or structural plates.
Especially problematic are panels of this type in which the space between the two slabs serves to receive an insulating layer and also as a trapped-air layer.
Panels of the latter type are particularly desirable in the assembly of prefabricated units as structural elements because they give better insulating capacity than so-called three-layer panels in which the space between the concrete slabs is entirely filled with thermal insulating solids.
The improved panels can be made in a first of the known methods by forming a first reinforced concrete slab with its reinforcement in a horizontal formwork bed, applying to this slab a so-called kubby foil while the surface of the slab is fresh, applying the insulator layer to the knubby foil and thereafter casting the other concrete slab with its reinforcement on the insulating layer.
This system has the disadvantage that the knubby foils are expensive and, because they are composed of synthetic resins, are generally flammable which may render the panels in violation of ordinances or codes barring the use of flammable materials.
In most cases, especially where the panel is to have a dead air space, combustible materials must be excluded. Knubby foils of nonflammable or noncombustible materials are so expensive that their use is impractical.
In a second process for fabricating panels with air spaces and insulating solids, the first reinforced concrete slab is made in the horizontal formwork and a separate foil is then applied, whereupon sand is deposited on this foil to the desired thickness of the air layer. The heat insulating material is then applied in the form of plates to the sand layer and the load-carrying inner concrete slab is then formed upon the insulating layer. The sand is removed. The air layer which should have the desired thickness, is then found to vary in thickness over the surfaces of the two slabs. Furthermore, one or the other of the slabs may be found to have thin spots. The static strength properties of such panels cannot readily be calculated and production of the panels is not reproducible. At some locations, it is not possible to satisfactorily embed the connecting anchor in the second slab, or even the first slab when the sand and insulating layers are applied while the first slab is still fresh or incompletely set.
The use of the sand has also been found to be costly and inconvenient and even when the panels are being erected, there may be significant amounts of sand which pour from the gap.
In a third method, the slabs are cast in formwork beds with openings in the lateral formwork.
Initially the lower or outer reinforced concrete slab is formed by pouring concrete around the reinforcement layered in the bed. Through the openings in the lateral formwork, conical planks are inserted and laid on the fresh concrete to serve as spacers. The heat insulating layer is then applied and the second slab is formed. After hardening of the concrete the conical blanks are withdrawn by hydraulic presses. Formwork beds of this type constitute expensive molds and the numerous additional manipulation to which the operation must be subject renders the panel production highly expensive.